Fuel cells have the potential to provide power for a wide variety of applications ranging from electronic devices to transportation vehicles. Cells operating with H2 and air as inputs and electric power and water as the only outputs are of particular interest because of their ability to produce power without degrading the environment. Polymer electrolyte membranes (PEMs), with hydrophilic, proton-conducting channels embedded in a structurally sound hydrophobic matrix, play a central role in the operation of polymer electrolyte fuel cells. PEMs are humidified by contact with air (the presence of water in PEMs is essential for proton transport). In addition, PEMs must transport protons to catalyst sites, which are typically crystalline solids such as platinum. The arrangement of the hydrophilic domains in the vicinity of both air and solid substrates is thus crucial. A University of California, Berkeley, and Berkeley Lab group has now provided the first set of data on morphology of PEMs at interfaces by a combination of x-ray scattering and microscopy.

Optimizing Hydrogen Fuel Cells

It’s no secret that the world's dependence on energy from coal, oil, and gas, up to now a driving force behind the industrial society we take for granted, poses challenges, owing to the nonrenewable nature of these energy sources, their environmental consequences up to and including global climate change, the financial challenges associated with their increasing scarcity and environmental effects, and the eventual probability of increased political conflict and instability. Using less energy (conservation) can contribute significantly, but alternative technologies that can provide energy that is plentiful, nonpolluting, and affordable are badly needed.

Among the many energy technologies under investigation in laboratories around the world is that of hydrogen fuel cells, which use hydrogen and air as fuels to generate electricity, with water as the only "waste" product. Superficially similar to batteries, these fuel cells consist of an electrode where hydrogen ions (protons) and electrons are produced, an electrolyte membrane that passes only the hydrogen ions, and another electrode where the protons are collected and recombine with the electrons that arrive there via an external wire after accomplishing the assigned task (e.g., running an electric motor). A major challenge to making efficient and thus economically viable fuel cells, is finding an electrolyte that is both durable and easily passes the protons. To this end, Park et al. have used x-ray scattering and electron microscopy to investigate microchannels that pass protons through polymer electrolytes.

Conceptual diagram of a polymer electrolyte hydrogen fuel cell. Hydrogen fuel is channeled through field flow plates to the anode on one side of the fuel cell, while oxidant (oxygen or air) is channeled to the cathode on the other side of the cell. At the anode, a platinum catalyst causes the hydrogen to split into positive hydrogen ions (protons) and negatively charged electrons. The PEM allows only the positively charged ions to pass through it to the cathode. The negatively charged electrons must travel along an external circuit to the cathode, creating an electrical current. At the cathode, the electrons and positively charged hydrogen ions combine with oxygen to form water, which flows out of the cell. Figure courtesy of Wikipedia.

The Berkeley group studied PEMs composed of block copolymers supported by a silicon substrate. Copolymers consist of at least two types of polymer structural units (monomers), which can be arranged in different ways. In block copolymers, a polymer composed of one monomer is linked to another polymer by covalent bonds. In the present case, the blocks were hydrophilic polystyrene sulfonate (PSS) (forming the proton-conducting channels) and hydrophobic polymethylbutylene (PMB) (serving as the matrix through which the channels run). Grazing-incidence small-angle x-ray scattering (GISAXS) data from ALS Beamline 7.3.3 provided information about the orientations of the channels near the air interface and through the interior of 180-nm-thick PEMs before and after exposure to humid air.

In the first sample studied, scattering at incident angles below the critical angle and thus dominated by contributions from the PEM/air surface contained well-defined spots, indicating the presence of hydrophilic channels oriented perpendicular to the surface. This morphology is ideal for water transport. In contrast, scattering at incident angles above the critical angle and thus containing contributions from the entire film, exhibited a scattering ring, indicating the presence of hydrophilic channels parallel to the plane of the film. The scattering ring arises because all orientations of the hydrophilic channels in the plane are equally likely. Transmission electron micrographs from the same sample confirmed the two morphologies determined by GISAXS. The parallel orientation, if it were to exist at the PEM/catalyst interface would lead to poor reaction kinetics, i.e., poor energy-delivery rates.

GISAXS patterns (intensity represented by color as a function of the scattering vectors qy and qz) for 180-nm-thick PSS–PMB film on a silicon substrate after exposure to humid air. Data were obtained at two incident angles, αi = 0.14° (below the critical angle αc), which measures the channel orientation near the air surface, and αi = 0.21° (above αc), which measures the channel orientation through the entire film.

Transmission electron microscopy image of the same sample. Coexisting perpendicular and parallel orientations of cylinders propagate from the air surface (stained with RuO4) and the silicon substrate, respectively, as shown schematically in the upper right inset. The higher-magnification TEM image shown in the bottom left inset indicates the presence of native SiO2 layers (bright) on top of the Si substrate and preferential wetting of PSS (dark) on substrate.

The interfacial morphologies depend crucially on molecular structure. The Berkeley group studied a second PSS–PMB copolymer that was identical to the first except that the concentration of sulfonic acid groups in the PSS block was doubled, thereby increasing its hydrophilicity. There was no difference in the bulk morphology of the two samples in the dry state, yet the interfacial properties of the samples were dramatically different. The highly sulfonated sample exhibited parallel hydrophilic cylinders at both air and silicon interfaces. This morphology may hinder water transport from the air because the hydrophilic channels in the PEM are buried beneath a hydrophobic skin.

Ordinarily one might assume that increasing the hydrophilicity of the PEM would lead to better water and proton transport, but these results suggest that this is not true. In the case of the PEM studied here, inappropriate orientation of the proton- and water-transporting channels with increasing sulfonation may lead to poorer performance. While these results demonstrate that one can obtain the orientation of the transporting channels, the relationship between morphology and ion transport is only suggestive and has not yet been determined. Future work will be geared toward determining this relationship.